1. Field of the Invention
The present invention relates to the use of coherent energy pulses, as from high power pulsed lasers, in the shock processing of solid materials, and, more particularly, to methods and apparatus for improving properties of solid materials by providing shock waves therein where the laser beam impacts the solid material at an oblique angle. The invention is especially useful for enhancing or creating desired physical properties such as hardness, strength, and fatigue strength.
2. Description of the Related Art
Known methods for the shock processing of solid materials, particularly, known methods for the laser shock processing of solid materials, typically used coherent energy from a laser beam oriented normal to a workpiece. This normal orientation, if shape of the laser beam is symmetrical, makes a symmetrical impact area on the workpiece.
In FIG. 1, laser shock processing is shown with the laser beam 10, having a particular diameter D, passing through a transparent overlay layer 12 to impact workpiece 14. The impact area of workpiece 14 is shown as the area of a spot having a diameter S1, shown in FIGS. 1A and 1B.
Particular constraints of laser shock processing are sometimes created based on the shape of the material or other geometric factors such as when attempting to work integrally bladed rotors (IBR's), blind bores, slots, or dovetail sections. In these cases, the laser beam 10, from necessity, needs to impact workpiece 14 at a particular angle .THETA.. This causes the dimensions of the impact spot to change in one or more directions. As shown in FIGS. 2a and 2b, the shape of the spot changes to an ellipse with a major diameter S2, even though the nominal beam diameter D of laser beam 10 has not varied.
The consequences of such a change of the incident spot shape necessarily changes the power density applied to the workpiece 14, all other factors being constant. This particular change of the applied pressure, i.e., the power density per unit area compared to other areas on the same surface creates a possibility of non-uniformly working the material, thereby possibly losing some of the benefits of laser shock peening.
Typical laser shock processing techniques and equipment can be found in U.S. Pat. No. 5,131,957 to Epstein, along with that of U.S. patent Ser. No. 08/547,012 entitled LASER PEENING PROCESS AND APPARATUS, assigned to the assignee of the present invention and hereby incorporated by reference.
This non-uniformity of power application to a workpiece may cause severe problems, particularly when hitting a workpiece from opposite sides at the same time, as used with a split beam laser system. Such opposing hits are sometimes needed on workpieces of thin cross-section, such as disks, blades, and other workpieces of different geometries. In conventional split beam processing, there is a possible effect of not having the laser processed portions on the opposite sides of the workpiece worked identically, and at the same time when elliptical spots are utilized. Such non-uniform working of the workpiece may cause over or under working of the material, or distortion of the work pieces, thereby not achieving the goals of laser shock processing.
Furthermore, based upon the oblique angle .THETA., along with the particular transparent overlay material utilized, polarization issues regarding the reflection of the laser beam from the surface of the transparent overlay layer can possibly degrade and reduce the power applied to the workpiece.
What is needed in the art is a way to modify the incident spot of laser applied power to consistently make and have a uniform working of the workpiece at such location.